1. Field of the Invention
This invention relates to a physical quantity detection device for detecting a physical quantity through resistance variation.
2. Description of the Prior Art
A detection circuit for a semiconductor pressure sensor using the piezoresistance effect is known. FIGS. 6A and 6B show examples of such a detection circuit. Japanese patent publication No. 2976487 discloses this detection circuit for a strain gage with temperature characteristic compensation.
The piezoresistance effect in diffusion resistors decreases with increase in the temperature, which decreases the sensitivity. On the other hand, the resistance increases. Particularly, the increase in the resistance depends on a density of impurity in the diffused resistors forming the strain gages. When the strain gages are driven with a constant current, the voltage applied to the strain gages increases with increase in the temperature. This provides compensation of decrease in the sensitivity in the sensor in accordance with the density of the impurity in the diffused resistors. This is the reason for using the circuits shown in FIGS. 6A and 6B.
In FIGS. 6A and 6B, when the diffused resistors Ra to Rd sense no strain (stress), that is, if no physical quantity is applied to this sensor, it is desirable that Ra=Rb=Rc=Rd. In this condition the output of the sensor xcex94Vout=0.
However, dispersion in manufacturing results in Raxe2x89xa0Rbxe2x89xa0Rcxe2x89xa0Rd, so that xcex94Voutxe2x89xa00. This is referred to as an offset voltage Voff.
In the condition that the offset voltage is developed, that is, xcex94Vout=Voffxe2x89xa00, if a constant current is applied to the strain gage, the larger offset voltage, the more the temperature characteristic in the offset voltage (hereinafter referred to as offset temperature characteristic) increases. Therefore, to compensate the offset temperature characteristic, it is necessary to add a separate compensation circuit.
Moreover, it is necessary to obtain data of the offset temperature characteristic with large variation of temperature during the manufacturing process and to adjust the offset temperature characteristic on the basis of the data.
Japanese patent publication No. 2976487 discloses an example of the prior art offset temperature compensation circuit as shown in FIG. 7.
In this circuit, the resistors R9, R10, R11 correspond to the offset temperature compensation circuit.
Prior to explanation of the offset temperature compensation circuit, operation of the whole circuit will be described.
The operational amplifier OP1 operates to equalize the voltage drop in the resistor R3 to that in the resistor R5. Therefore, if a resistor having a temperature coefficient of resistance (TCR) of almost zero is used as the resistor R5, a current Io flowing through the bridge circuit including gage resistors Ra to Rd is substantially constant, though the temperature varies.
Here, using diffused resistors including boron as the gage resistors Ra to Rd and making the density of the p type impurity in the gage resistors Ra to Rd about 1020 cmxe2x88x923 provides temperature compensation to sensitivity. This corresponds to the sensitivity-temperature compensation circuit.
The operational amplifier OP2 and the operational amplifier OP3 connected to a resistor R6 are used as voltage follower circuits to convert the bridge output voltage into a current with the resistor R6. The current is supplied to an operational amplifier OP4 through transistors Tr1 and Tr2 having Darlington connection. The operational amplifier OP4 amplifies the current. The resistor R8 connected to the input of the operational amplifier OP4 is used for zero point adjustment.
The offset voltage of the bridge circuit can be made zero by laser-trimming the resistor R1 or R2 which is formed with CrSi thin film resistor having an almost zero TCR. However, because its TCR is largely different from those of the gage resistors Ra to Rd (about 1600 ppm/xc2x0 C.), the offset voltage varies with the temperature.
In this circuit, operation of the resistors R9, R10, R11 for offset temperature compensation will be described. Here, these resistors have a relation R9=R10 less than  less than R11. This condition makes the current flowing through the resistor R11 constant though the temperature varies.
At first, because the bridge circuit is driven with a constant current, the applied voltage thereto varies at a TCR which is the same as that of the gage resistors Ra to Rd. Accordingly, the voltage potential V6 decreases with increase in temperature. On the contrary, the voltage difference between the voltage potentials V6 and Vd increases.
Therefore, laser-trimming the resistor R9 to have the voltage Vf relatively approached the ground potential decreases the current flowing through the resistor R11 with increase in temperature. On the other hand, laser-trimming the resistor R10 to have the potential Vf approached the potential Vd increases the current flowing through the resistor R11 with increases in temperature. That is, trimming the resistor R9 or R10 provides a temperature characteristic to the current flowing through the resistor R11. This temperature characteristic compensates the temperature characteristic in the bridge output. The offset temperature characteristic can be compensated in this way.
However, this operation requires measurement at the room temperature and at a high temperature for every circuit and laser trimming to obtain a target resistance which is calculated for the desired potential of Vf.
The aim of the present invention is to provide a superior physical quantity detection circuit.
According to the present invention, a first aspect of the present invention provides a physical quantity detection device comprising: a first sensing resistor having a first resistance varying in accordance with a first physical quantity relating a detection physical quantity; a second sensing resistor having a second resistance varying in accordance with a second physical quantity relating said detection physical quantity, first ends of said first and second sensing resistors being connected to a first voltage potential; a first current source for flowing a first constant current through said first sensing resistor, a first end of said first current source being connected to a second end of said first sensing resistor; a second current source for flowing a second constant current through said second sensing resistor, a first end of said second current source being connected to a second end of said second sensing resistor, second ends of said first and second current sources being connected to a second potential which is different from said first voltage potential; and outputting means for outputting a voltage difference signal indicative of said detection physical quantity between the second ends of said first and second sensing resistors.
According to the present invention, a fourth aspect of the present invention provides a physical quantity detection device based on the third aspect, wherein second ends of said first and second resistors are connected to each other at a junction point, said physical quantity detection device further comprising a third resistor connected to said junction point, wherein said first and second resistors are connected to said second potential through said third resistor, and a voltage potential of said junction point of said first and second resistors is supplied to an input of said operational amplifier.
According to the present invention, a fifth aspect of the present invention provides a physical quantity detection device based on the third aspect, wherein second ends of said first and second resistors are connected to each other at a junction point, and a voltage potential of said junction point is supplied to an input of said operational amplifier.
According to the present invention, a fifth aspect of the present invention provides a physical quantity detection device based on the third aspect, wherein one ends of said first and second resistors are connected to each other at a junction point, and a voltage potential of said junction point is supplied to an input of said operational amplifier.
According to the present invention, a sixth aspect of the present invention provides a physical quantity detection device based on the third aspect, wherein either of said first or second resistors is trimmed.
According to the present invention, a seventh aspect of the present invention provides a physical quantity detection device based on the first aspect, wherein said first current source includes a first control element connected to said first sensing resistor in series, and a first resistor connected to said first control element in series, and a first amplifier for generating a first control voltage supplied to said first control element on the basis of a first reference voltage, and said second current source includes a second control element connected to said second sensing resistor in series, a second resistor connected to said second control element in series, and a second amplifier for generating a second control voltage supplied to said second control element on the basis of a second reference voltage.
According to the present invention, an eighth aspect of the present invention provides a physical quantity detection device based on the seventh aspect, wherein a voltage potential at a first junction point between said first control element and said first resistor is supplied to an input of said first operational amplifier and a voltage potential at a second junction point between said second control element and said second resistor is supplied to an input of said second operational amplifier.
According to the present invention, a ninth aspect of the present invention provides a physical quantity detection device comprising: a first circuit including a first sensing resistor and a first constant current source connected to said first sensing resistor in series through a first junction point, said first sensing resistor having a first resistance varying in accordance with a first physical quantity relating to a detection physical quantity, said first constant current source flowing a first constant current through said first sensing resistor; a second circuit including a second sensing resistor and a second constant current source connected to said second sensing resistor in series through a second junction point, said second sensing resistor having a second resistance varying in accordance with a second physical quantity relating said detection physical quantity, said second constant current source flowing a second constant current through said second sensing resistor, first ends of said first and second circuits being connected to a first potential, second ends of said first and second circuits being connected to a second potential which is different from said first potential; and outputting means for outputting a voltage difference signal indicative of said detection physical quantity between said first and second junction points.
According to the present invention, a tenth aspect of the present invention provides a physical quantity detection device based on the ninth aspect, wherein said first and second current sources have substantially the same temperature coefficients in said first and second constant currents, respectively.
According to the present invention, an eleventh aspect of the present invention provides a physical quantity detection device based on the ninth aspect further comprises an operational amplifier for generating a control voltage on the basis of a reference voltage, said operational amplifier being shared by said first and second current sources, wherein said first current source further includes a first control element and a first resistor for flowing said first constant current on the basis of said control voltage, and said second current source further includes a second control element and a second resistor for flowing said second constant current on the basis of said control voltage.
According to the present invention, a twelfth aspect of the present invention provides a physical quantity detection device based on the eleventh aspect wherein second ends of said first and second resistors are connected to each other at a junction point, said physical quantity detection device further comprising a third resistor connected to said junction point, wherein said first and second resistors are connected to said second potential through said third resistor, and a voltage potential of said junction point of said first and second resistors is supplied to an input of said operational amplifier.
According to the present invention, a thirteenth aspect of the present invention provides a physical quantity detection device based on the eleventh aspect, wherein second ends of said first and second resistors are connected to each other at a junction point, and a voltage potential of said junction point is supplied to an input of said operational amplifier.
According to the present invention, a fourteenth aspect of the present invention provides a physical quantity detection device based on the eleventh aspect, wherein either of said first or second resistors is trimmed.
According to the present invention, a fifteenth aspect of the present invention provides a physical quantity detection device based on the ninth aspect, wherein said first current source includes a first control element connected to said first sensing resistor in series, and a first resistor connected to said first control element in series, and a first amplifier for generating a first control voltage supplied to said first control element on the basis of a first reference voltage, and said second current source includes a second control element connected to said second sensing resistor in series, a second resistor connected to said second control element in series, and a second amplifier for generating a second control voltage supplied to said second control element on the basis of a second reference voltage.
According to the present invention, a sixteenth aspect of the present invention provides a physical quantity detection device based on the fifteenth aspect, wherein a voltage potential at a first junction point between said first control element and said first resistor is supplied to an input of said first operational amplifier and a voltage potential at a second junction point between said second control element and said second resistor is supplied to an input of said second operational amplifier.
According to the present invention, a seventeenth aspect of the present invention provides a method of adjusting an offset voltage in a physical quantity detection device comprising: a first sensing resistor having a first resistance varying in accordance with a first physical quantity relating a detection physical quantity; a second sensing resistor having a second resistance varying in accordance with a second physical quantity relating said detection physical quantity, first ends of said first and second sensing resistors being connected to a first voltage potential; a first current source for flowing a first constant current through said first sensing resistor, a first end of said first current source being connected to a second end of said first sensing resistor; a second current source for flowing a second constant current through said second sensing resistor, a first end of said second current source being connected to a second end of said second sensing resistor, the second ends of said first and second current sources being connected to a second potential which is different from said first voltage potential; and outputting means for outputting a voltage difference signal indicative of said detection physical quantity between the second ends of said first and second sensing resistors with said offset voltage, said method comprising the steps of: measuring said offset voltage at a temperature within a usable temperature range of said physical quantity detection device; and adjusting said first and second constant currents on the basis said measured offset voltage.
According to the present invention, an eighteenth aspect of the present invention provides a method of adjusting an offset voltage in a physical quantity detection device comprising: a first sensing resistor having a first resistance varying in accordance with a first physical quantity relating a detection physical quantity; a second sensing resistor having a second resistance varying in accordance with a second physical quantity relating said detection physical quantity, first ends of said first and second sensing resistors being connected to a first voltage potential; a first current source for flowing a first constant current through said first sensing resistor, a first end of said first current source being connected to a second end of said first sensing resistor, said first current source includes a first control element and a first resistor for flowing said first constant current on the basis of a control voltage; a second current source for flowing a second constant current through said second sensing resistor, a first end of said second current source being connected to a second end of said second sensing resistor, said second current source includes a second control element and a second resistor for flowing said second constant current on the basis of said control voltage; an operational amplifier shared by said first and second constant current sources for generating said control voltage on the basis of a reference voltage, wherein a difference voltage between a first junction point between said first sensing resistor and said first current source and a second junction point between said second sensing resistor and said second current source has an offset voltage, said method comprising the steps of: measuring said offset voltage; and trimming either of said first or second resistors to adjust said offset voltage.
According to the present invention, a nineteenth aspect of the present invention provides a method of adjusting an offset voltage in a physical quantity detection device comprising: a first sensing resistor having a first resistance varying in accordance with a first physical quantity relating a detection physical quantity; a second sensing resistor having a second resistance varying in accordance with a second physical quantity relating said detection physical quantity, first ends of said first and second sensing resistors being connected to a first voltage potential; a first current source for flowing a first constant current through said first sensing resistor, a first end of said first current source being connected to a second end of said first sensing resistor, said first current source including a first control element and a first resistor for flowing said first constant current on the basis of a first control voltage, and a first operational amplifier for generating said first control voltage on the basis of a first reference voltage; a second current source for flowing a second constant current through said second sensing resistor, a first end of said second current source being connected to a second end of said second sensing resistor, said second current source including a second control element and a second resistor for flowing said second constant current on the basis of a second control voltage, and a second operational amplifier for generating said second control voltage on the basis of a second reference voltage, wherein a difference voltage between a first junction point between said first sensing resistor and said first current source and a second junction point between said second sensing resistor and said second current source indicates said detection physical quantity wherein said difference voltage includes an offset voltage, and the difference voltage is equal to the offset voltage when the detection physical quantity is not detected by the physical quantity detection device, said method comprising the steps of: measuring said offset voltage; and controlling said first and second reference voltages to adjust said offset voltage on the basis of the measuring.